This application claims priority under 35 U.S.C. xc2xa7119 to Great Britain Patent Application No. 0025734.5 filed Oct. 19, 2000 and Great Britain Patent Application No. 0031857.6 filed Dec. 30, 2000.
Not Applicable.
Not Applicable.
1. Field of the Invention
The present invention relates to a heating cable for use in electric trace heating applications.
2. Description of the Related Art
Trace heating cables fall into two general categories, that is parallel resistance cut-to-length types and series resistance fixed length types.
In parallel resistance type cables, generally two insulated conductors (known as buswires) extend longitudinally along the cable. A resistance heating wire is spiraled around the conductors, electrical connections being made alternately at intervals along the longitudinally extending conductors. This creates a series of short heating zones spaced apart along the length of the cable. The heating wire must be selectively insulated from the conductors and also encased within an insulating sheath. Available parallel trace heating cables either use polymeric external insulation sheaths which limit the use of such cables to maximum temperatures of for example 250xc2x0 C., or use glass insulation for the external sheath which can operate at higher temperatures, for example above 400xc2x0 C., but which are not waterproof.
Series resistance heaters must be specifically designed so that the power produced meets the requirements for a particular length of cable. This is not convenient and represents a major constraint. Generally series heaters include longitudinally extending resistance wires embedded in a mineral insulation which can withstand high temperatures. A typical construction comprises two ni-chrome heating conductors, magnesium oxide powder insulation, and an outer stainless steel sheath. The whole construction may be drawn down from an outside diameter of typically 80 mm to an outside diameter of 4 mm at which point the heater is flexible to enable it to be installed relatively easily and has an electrical resistance producing a desired output per unit length. Unfortunately the available range of resistances is limited and, particularly, short lengths (typically less than 10 meters) with appropriate low power outputs are not available.
In summary, parallel heaters are convenient in use but are not available in forms which combine both a high temperature withstand and a waterproof construction, whereas series heaters are available which can withstand high temperatures and are waterproof but cannot be cut to length and therefore must be designed specially to fit particular applications and are difficult to design for use in short lengths.
It is an object of the present invention to obviate or mitigate the problems outlined above.
According to the present invention, there is provided a mineral insulated heating cable comprising two electrical conductors extending along the length of the cable and an array of heating elements distributed along the lengths of the cable and connected in parallel between the conductors, wherein each conductor is encased in an inner sheath of insulating material through which connections are made to each heating element, the inner sheaths and heating elements are encased in an outer sheath of insulating material and the outer sheath is covered by a metal jacket extruded around the outer sheath.
The term xe2x80x9cmineral insulatedxe2x80x9d is used herein to indicate a heating cable in which all components can withstand long-term exposure to high temperatures, e.g. 250xc2x0 C. and above. In such cables, insulation could be formed from for example tape manufactured from glass and/or mica.
The invention is based on the realization that with careful process control it is possible to extrude a jacket of for example aluminum onto a preformed trace heating cable of the parallel resistance type, the aluminum sheath making the overall assembly waterproof and therefore enabling the use within the cable of components which themselves do not have to be waterproof. A waterproof structure which can withstand high temperatures results.
The conductors and the inner sheaths may be encased in an intermediate sheath of insulating material through which connections are made between each conductor and each healing element, the intermediate sheath may be formed from glass tape which may be coated with a stabilizer.
The conductors may be nickel plated copper, the heating elements may be formed from a ni-chrome resistance heating wire spiraled around the conductors, and the resistance heating wire may be in contact with the conductors through openings in the inner sheath such that the wire touches the conductors, a positive electrical connection being made between the conductors and the wire by sprayed metal. Metal may be sprayed onto the conductors both before and after positioning of the heating wire.
Each inner sheath may be formed from mica tape and the outer sheath may also comprise mica tape. The outer sheath may also comprise glass tape which may be coated with a stabilizer. The stabilizer may be for example silicone varnish to provide initial waterproofing, or a ceramic fiber adhesive incorporating a rigidizer and hardener.
The metal jacket may be of oval section to improve overall flexibility of the product.